C.S. Lewis returned to Christianity in 1931.
Lewis had moved away from religious belief during his adolescence and early adulthood. His intellectual journey back toward Christianity developed through reading, conversation, and reflection. Important influences included his friendship with J.R.R. Tolkien and discussions with other Christian-minded writers at Oxford.
Lewis described his conversion as a gradual process rather than a single simple moment. In 1931, he became a Christian, and he later became one of the twentieth century’s best-known Christian apologists through books such as Mere Christianity, The Problem of Pain, and Miracles.
Accounts sometimes distinguish between Lewis’s return to belief in God in 1929 and his later embrace of Christianity in 1931. That distinction explains why different summaries mention both years. For the question of his return to Christianity specifically, 1931 is the standard answer.